Chlorine dioxide (ClO2) is a relatively small, volatile, and versatile free radical molecule with bleaching, oxidizing, and deodorizing properties, as well as antimicrobial properties, namely, bactericidal, viricidal, sporicidal, algicidal, and fungicidal properties. It is frequently used to control microorganisms on or around food products because chlorine dioxide destroys the microorganisms without producing concentrations of byproducts that would pose a significant adverse risk to human health. Examples of these adverse byproducts that may be produced by other types of oxidants include chloramines and chlorinated organic compounds. Chlorine dioxide's physiological mode of destroying microbes has been attributed to the destruction of cell walls and cell membranes and disrupting transport of nutrients from the external environment into microorganisms.
Chlorine dioxide has also long been recognized for treatment and reduction of odors caused by compounds and microorganisms through an oxidation process. Examples of odor-causing compounds include: sulfur-containing compounds (hydrogen sulfide, mercaptan sulfides, organic disulfide, sulfoxides, etc.), oxygen containing compounds (phenols, aldehydes, aliphatic alcohols, etc.) and some nitrogen containing compounds (tertiary and secondary amines, etc.). A low concentration of chlorine dioxide, in either gaseous or liquid state, is effective for most antimicrobial and deodorization applications. In addition to chlorine dioxide, other compounds, such as peracetic acid, bromine, sulfur dioxide, and carbon dioxide also exhibit antimicrobial and disinfecting properties.
As an oxidizing biocide, chlorine dioxide can also be used as a disinfectant or as an oxidant in water treatment systems. It may be used in both the pre-oxidation and the post-oxidation stages of the treatment as a primary oxidant or as a primary or secondary disinfectant. By adding chlorine dioxide in the pre-oxidation phase of the purification of surface water, the growth of bacteria and algae can be controlled in the subsequent phases of the treatment. Chlorine dioxide also acts as an oxidant to the colloidal substances, aiding in the coagulation process and improving the removal of turbidity.
In its vapor state, chlorine dioxide is not stable during storage and can be explosive at concentrations above about 10% in dry air. At temperatures lower than −40° C., chlorine dioxide vapor can be compressed to a liquid state to reduce the risk of explosion; above this temperature, however, highly concentrated liquid chlorine dioxide can be explosive. Therefore, chlorine dioxide vapor or liquid is usually not produced and shipped under pressure. Instead, chlorine dioxide is typically generated at the point of use via conventional chlorine dioxide generators or other means. Conventional chlorine dioxide generation can be carried out in an efficient manner for large-scale operations to produce chlorine dioxide by reacting sodium chlorite or sodium chlorate solution with an acid such as sulfuric acid, hydrochloric acid, or a reducing agent. Chlorine dioxide can also be generated by one of the following reactions: mixing sodium chlorite solution with a strong chlorine solution at low pH, mixing a sodium chlorite solution with chlorine gas at near neutral pH under a vacuum, or reacting solid sodium chlorite in a sealed reactor cartridge with humidified chlorine gas flowing through it. All of the processes mentioned above require expensive generation equipment, high maintenance costs, and highly trained and skilled workers to operate the equipment in a safe manner. As a result, the use of such generators has been limited to the fields of poultry processing, pulp and paper bleaching, and water treatment facilities, where the high capital and operating cost of the generators can be justified by the large consumption of chlorine dioxide.
In addition to the methods discussed above, chlorine dioxide can also be generated by the electrolysis of sodium chlorite solutions. This process requires electricity to operate the electrolytic equipment, and high maintenance efforts to ensure the efficiency of the equipment. The electrolysis process not only produces less chlorine dioxide compared to conventional generators, but special sodium chlorite solutions are also required for this process to reduce the level of suspended solids and scaling that can clog the electrolytic cell. Further, proportional amount of wastes, such as sodium hydroxide solution, are produced along with chlorine dioxide during the electrolytic reaction.
A solution of a metal chlorite and water where the pH of the solution is maintained at 8 or above is sometimes referred to as a “stabilized chlorine dioxide” solution. Applications requiring small quantities of chlorine dioxide can be approached by the use of “stabilized chlorine dioxide”, which generally refers to sodium chlorite, a reactant of chlorine dioxide. Sodium chlorite by itself only has bacteriostatic properties (inhibits rather than killing bacteria) and does not provide complete disinfection. Some claims have been made to the use of sodium chlorite as a bactericide in situations in which bacteria can provide the necessary acidity for the “activation” step to produce chlorine dioxide. In any case, the amount of chlorine dioxide produced under these conditions is insignificant. In most other cases, “stabilized chlorine dioxide” still requires the activation step of reacting a sodium chlorite solution with an acid. The pH of the reacting solution must be lowered to below 5, typically to a pH range between about 2 to about 3, in order to produce chlorine dioxide according to the following equation:5 ClO2−+5 H+→4 ClO2+HCl+2H2OThis approach, or any other type of “two-part system,” is usually performed at the application site, requiring trained personnel to properly activate the product. In addition, the use of “stabilized chlorine dioxide” requires mixing equipment and manipulation of potentially dangerous acids, thus exposing users to inadvertent skin contact and inhalation of acid vapors. Transportation of the “stabilized chlorine dioxide” also involves large volumes of water, resulting in a costly and difficult operation for remote and/or disaster recovery uses.
Attempts have also been made to manufacture devices that produce chlorine dioxide using a mixture of solid sodium chlorite and acidulant in solid forms (e.g. citric acid, sodium bisulfate, organic anhydride, etc.). These devices usually require complicated formulation processes, one of which involves drying individual reactants to lower their water content and mixing the dried reactants in the presence of desiccant materials (e.g. calcium chloride), to prevent the premature generation of chlorine dioxide that is initiated by atmospheric moisture. Such processes must take place in specially-designed environments that minimize moisture contact with mixed reactants during the formulation/packaging process. In addition, a protective barrier is required to prevent the contact of atmospheric moisture with the mixed reactants prior to use. In the presence of water/moisture, these devices generate chlorine dioxide solution or chlorine dioxide vapor. Due to the nature of the manufacturing process, these devices usually involve high manufacturing costs.
Many compositions and methods for generating chlorine dioxide solutions are known in the art. For example, U.S. Pat. No. 2,022,262 discloses stable stain-removing compositions made from a dry mixture of water-soluble alkaline chlorite salt, an oxalate, and an acid. U.S. Pat. No. 2,071,091 discloses the use of chlorous acid and chlorites to kill fungi and bacterial organisms by exposing the organisms to the compounds at a pH of less than about 7. The patent also discloses using dry mixtures of chlorites and acids to produce stable aqueous solutions useful as bleaching agents. U.S. Pat. No. 2,482,891 discloses stable, solid, substantially anhydrous compositions comprising alkaline chlorite salts and organic acid anhydrides, which release chlorine dioxide when contacted with water. U.S. Pat. No. 2,071,094 discloses deodorizing compositions in the form of dry briquettes formed of a mixture of soluble chlorite, an acidifying agent, and a filler of relatively low solubility. Chlorine dioxide is generated when the briquettes contact water. U.S. Pat. No. 4,585,482 discloses a long-acting biocidal composition comprising a microencapsulated mixture of chlorite and acid that when added to water releases chlorine dioxide. The primary purpose of the microencapsulation is to provide for hard particles that will be free flowing when handled. The microencapsulated composition also protects against water loss from the interior of the microcapsule. The microcapsules produce chlorine dioxide when immersed in water. The microcapsules release chlorine dioxide relatively slowly and are therefore not suitable for applications that require the preparation of chlorine dioxide on a relatively fast basis. U.S. Pat. No. 6,063,425 discloses a method for disinfecting a meat carcass by spray application of an aqueous solution containing from about 0.05 to 0.12% of a metal chlorite and a sufficient quantity of an acid having a pKa of from about 2.0 to 4.4 to adjust the pH of the aqueous solution to about 2.2-4.5. This adjustment helps to maintain the chlorite ion concentration in the form of chlorous acid to not more than about 35% by weight of the aqueous solution, the molar ratio of the acid to metal chlorite being at least equal to the first pKa of the acid multiplied by the grams/liter concentration of metal chlorite in the aqueous solution.
Many devices and methods for producing chlorine dioxide solution are also known in the art. For example, Canadian Patent No. 959,238 discloses using two water-soluble envelopes, one containing sodium chlorite and the other containing an acid, to generate chlorine dioxide solution. The envelopes are placed in water and the sodium chlorite and acid dissolve in the water and react to produce a chlorine dioxide solution. PCT Application PCT/US98/22564 (WO 99/24356) discloses a method and device for producing chlorine dioxide solutions wherein sodium chlorite and an acid are mixed and enclosed in a semi-permeable membrane device. When the device is placed in water, water penetrates the membrane. The acid and sodium chlorite dissolve in the water and react to produce chlorine dioxide. The chlorine dioxide exits the device through the membrane into the water in which the device is immersed producing a chlorine dioxide solution that can be used as an anti-microbial solution or for other purposes. The primary disadvantage of the disclosed device and method is that ambient moisture can penetrate the semi-permeable membrane and initiate the reaction prematurely.
In general, the above prior art devices and methods using membranes are susceptible to premature activation by water or water vapor and therefore have a reduced shelf life unless sufficient steps are taken to protect the devices from exposure to ambient moisture or water. In addition, such devices and methods are typically slow to interact with water and produce the desired chlorine dioxide.
U.S. Pat. No. 6,764,661 discloses a device for producing an aqueous chlorine dioxide solution when placed in water that solves the aforementioned problem. One of the advantages of this device is that it is not susceptible to activation by ambient moisture. The device includes a membrane shell that defines a compartment, which includes one or more dry reactants (e.g., a metal chlorite and an acid) capable of producing chlorine dioxide when exposed to water. The device is provided with wick means extending between the outer membranes, creating two inner compartments. The wick means absorb water and transport it into the compartment by capillary action. Once water has entered the inner compartments, the reactant(s) in the compartments dissolve in the water and produce chlorine dioxide in an aqueous solution. This device is generally used to produce an aqueous solution, having antimicrobial and disinfecting properties, when exposed to water. It is not designed to produce a vapor having antimicrobial and disinfecting properties.
In some water treatment operations, the process of coagulation is employed to separate suspended solids from water. In general, finely dispersed solids (colloids) suspended in water are stabilized by negative electric charges on their surfaces, causing them to repel each other. This repulsion prevents the colloids from colliding and forming larger masses, called “flocs”. Coagulation promotes the aggregation of these colloidal particles to form flocs by destabilizing their surface charges. For example, cationic coagulants bond to negatively charged particles, thus lowering the charge and energy required to bring the colloidal particles into contact. As a result, the particles collide and form flocs.
Flocculation is the action of forming bridges between the flocs and binding the flocs into large agglomerates or clumps. Bridging occurs when segments of polymer chains of the flocs adsorb onto different particles, causing the flocs to aggregate. An anionic flocculant will react against a positively charged suspension, adsorbing on the particles and causing destabilization either by bridging or charge neutralization. Once suspended flocs are flocculated into larger flocs, they can usually be removed from the liquid by sedimentation, provided that a sufficient density difference exists between the suspended flocs and the liquid. The flocs can also be removed or separated by media filtration, straining, sedimentation, or floatation. Flocculation not only increases the size of the floc particles, but it also affects the physical nature of the flocs, making these particles less gelatinous and thereby easier to remove.
U.S. Pat. No. 5,023,012 discloses a composition for the purification of water, including a suitable coagulant for coagulating solid impurities dispersed in the water to form flocs and an organic hydrophilic colloid capable, when dispersed in the water, of absorbing large quantities of water to form a sol for aggregating the flocs. The proportion of organic hydrophilic colloid in the composition is such that when the composition is used to purify the intended quantity of water, the organic hydrophilic colloid does not interfere with coagulant dispersal in the water or with floc formation.
The aforementioned coagulating and purifying invention uses chlorine as a disinfectant. Recently, concern over the disinfection byproducts of chlorine has led to increased use of alternative disinfectants that produce safer byproducts. Chlorine reacts with organic matter in water to form trihalomethanes (THMs) and haloacetic acids (HAAs) which have been found to be carcinogenic.
The present invention provides systems and methods for disinfection and purification that overcome the aforementioned disadvantages and problems.